Video transcript

- We've talked a lot about ATP being the energy currency of cells, but I want to dig a little bit
deeper into that in this video. And as we'll see when
we go from ATP to ADP, ADP + a Phosphate group, we
have a release of free energy. If we look at just the system, ATP's free energy is over
here, but once hydrolysis has taken place and now it's
ADP + a Phosphate group, the free energy has dropped
by roughly 30.5 KJ/mol our Delta G is -30.5 Kilojoules per mole and if you watch the
videos on Gibbs Free Energy this tells us that this
is a spontaneous reaction. Delta, Delta G is less than zero, which tells us that this
is going to be spontaneous. Now when I first learned this I was like, well if it's going to be
spontaneous why wouldn't all of the ATP just spontaneously
and all of the water turn into ADP and just release it's energy as heat or whatever else? And the key is, it has
to get over this hump. You have this activation energy, it has to go uphill a little bit if there's no enzyme to catalyze it. And the reason why we have
this uphill hump is the way that ATP gets broken
up is that you have to, if we're talking about a water
molecule doing the hydrolysis which is typically what
people think about hydrolysis, although it can sometimes be
done by a different molecule, if you think about the water molecule what needs to happen is, you need this lone pair of electrons on this oxygen to be able to do what we
call a nucleophilic attack on this Phosphorous in
this phosphate group and if that happens, it forms this bond and then these electrons can
be taken back by this oxygen which gives its negative
charge right over there. Now you might say, "Okay,
this makes a lot of sense", but you have to rememeber
this, electrons are negative and they're surrounded by
these negative charges. So they have to overcome
getting close to these things so as they're approaching
these negative charges, they want to repel each other,
so you have to overcome that. And the way that it's
overcome is a class of enzymes called ATPases, ATPase. And what they do is,
remember these enzymes are these big protein
complexes and the ATP molecule combined in the right place. And they essentially try and
surround the ATP molecule with some positive ions. So let's say there's a positive ion here. So it can keep these electrons
busy while the water, or whatever is doing
the nucleophilic attack, can not have to worry about
these electrons over here. So it might have a positive ion here. And remember, if we think
in three dimensions, this thing is all wrapped
around in different ways around the ATP molecule,
so this is the ATPase. And so by having the enzyme over here, you're lowering the activation energy. And so it might end up looking
something more like this. And so the reaction can actually occur. So the reason why you don't
just see this happening all the time without an enzyme is why you have to overcome this hump. But once you have the enzymes
they can actually allow the reaction to occur. So this attacks this, forms this
bond, and then you say well, "You have another
hydrogen right over here, you have another hydrogen
right over here." But this could get nabbed
up by another water molecule becoming a hydronium ion, which is actually what's going to happen. And then, this character takes
away this pair of electrons, becomes a negative charge,
and we're left with that Phosphate group has
been broken off and then we have our ADP and we
have our release of energy. Now, typically you don't
want this to release energy for no good reason. Most ATPases are going
to leverage that energy that release this reaction
to phosphorylate a molecule. And in this case you can
kind of think that this hydroxide has been phosphorylated, but it might phosphorylate something else. Or it might change the
confirmation of this ATPase so it can do some other type of work, transfer molecules against
their concentration gradient or chemoelectric gradient,
whatever it might be. You don't want to just release the energy. Sometimes it might happen
if you just want to generate some heat, but
usually you're going to do some useful work for the actual cells. Hopefully this gives you a little more appreciation for ATP and
the reaction by which the hydrolysis can occur. This is called hydrolysis
because, at least in this example we're using a water
molecule to break it up or to take off a phosphate group.